This invention relates to an injection mold for forming from a synthetic resin a polygonal vessel closed at its bottom and having undercuts formed on its inner and outer surfaces.
A conventional mold for manufacture of a molded article of this type is constituted by a stationary molding plate, movable molding plate, a plurality of sidewalls provided between these plates, a core having slanting surfaces, disposed inside the sidewalls and integral with or attached to the movable molding plate, and slide cores sliding on the slanting surfaces of the former core in engagement therewith. In order to mold, by using a mold of this type, an article having undercuts on its inner and outer surfaces, the core is removed inward while the sidewalls are moved outward so as to be moved away from the undercuts. For this reason, mechanisms for releasing the mold from such undercuts have been adopted for example as disclosed in U.S. Pat. No. 3,905,740. There are examples of a mechanism for releasing sidewalls such as one in which an inclined pin is secured to the stationary molding plate and sidewalls are released in such a manner that, when the movable molding plate and the sidewalls are simultaneously moved at the time of mold opening, the sidewalls are pushed outward by the inclined pins, and one in which a cylinder which serves as a power source for moving a sidewall outward and an inclined pin are attached to the stationary molding plate so as to enable the sidewalls release under the guidance of the inclined pina. In an example of the latter type adapted for the mold designed to simultaneously release a plurality of sidewalls from undercuts, the number of cylinders employed is smaller than the number of sidewalls, and the sidewalls are interconnected by connection pins or linking plates. In another example, a spacer is mounted between the movable molding plate and a mount plate attached to a movable die plate of the body of the injection molding apparatus so as to form a space in which an ejection plate connected to an ejection rod of the body of the injection molding apparatus and made to advance or retract, and the ejection plate and sidewalls having slanting surfaces are connected by links, thereby pushing forward the ejection plate and outwardly releasing the sidewalls along slanting surfaces of reinforcement frames outside the sidewalls.
A mechanism for releasing the slide cores from a molded product is constructed in such a manner that an ejection plate is disposed between a mount plate and a movable molding plate in a manner similar to that in the case of the aforementioned sidewall releasing mechanism, this ejection plate being connected by links to slide cores capable of being pushed forward so that the slide cores are made to slide along slanting surfaces of the core, thereby releasing the mold.
As described above, mechanisms for moving the sidewalls and the slide cores away from undercuts have been adopted. In general, the operation of such mechanisms resides in that the slide cores are moved after the sidewalls have been moved. However, in an example such as disclosed in U.S. Pat. No. 3,905,740, an ejection plate is provided between a movable molding plate and a mount plate attached to a movable die plate of an injection molding machine, and this ejection plate is connected to slide cores and sidewalls by links so that the sidewalls and the slide cores are moved simultaneously.
There are various problems of the conventional injection molds due to the above-described structures, as described below. If the sidewalls and the slide cores are separately moved, that is, the slide cores are moved after the sidewalls have been moved, the molding cycle is lengthened and the productivity is lowered. Since, if the sidewalls are moved by the inclined pins alone, excessive forces are applied to the inclined pins, it is necessary to open the mold at a low speed. This is also undesirable because the molding cycle is lengthened. The mechanism in which the ejection plate is provided and is connected to the sidewalls or the slide cores by links necessitates a space for advancing or retracting movement of the ejection plate, increasing the entire size and weight of the mold as well as making the link mechanism complicate and increasing the number of parts. The failure occurring rate is thereby increased, which is undesirable in terms of maintenance.